True under-eye bags are outpouchings of the orbital fat in the lower eyelids.
In the younger population, eye bags are often solely the results of these outpouchings, with or without tear troughs. Whereas in the older population, these orbital fat outpouchings are often associated with lax skin, muscle, and tear troughs, and malar festoons.
Some eye bags, however, are not real. These “fake” eye bags exist, and thus a careful diagnosis of eye bag is critical to the proper selection of treatment options.
One common condition of “fake” eye bags is hypertrophic or enlarged pretarsal muscle under the eyelash in the lower eyelids, often becoming exaggerated when smiling.
Another “fake” eye bags is tear trough deformity. Tear trough is a sunken trough area of volume loss below the lower eyelids. Tear trough is often associated with eye bags.
Another “fake” eye bags are malar festoons, which are localized squishy swelling on the cheek bone below the orbits.
In the older population, eye bags are caused by the weakening of orbital septum and the increasing descent and flattening of cheeks. The orbital septum is a fibrous wall which prevents fat herniation, but as we get old, this wall becomes weaker and allows more and more fat to bulge outward.